Ostracode Biostratigraphy of Shiranish, Hartha and Mushorah Formations from Selected Boreholes Northwest and Central Iraq

Ostracode biostratigaphy were investigated in detail from three formations (Shiranish, Hartha and Mushorah ) in subsurface sections of four boreholes Northwest and centre of Iraq. On the basis of the ostracode species distribution, the studied formations were divided into four ostracode biozones as follows: Biozone 1: Cytherella IRC 22 Interval zone. (Early Campanian). Biozone 2: Occultocythereis elongata Interval zone. (Late Campanian). Biozone 3: Krithe sp. M 1113 Interval zone. (Early Maastrichtian). Biozone 4: Holcopocythere bassiporosa Interval zone. (Late Maastrichtian). Depending on the above biozones correlated with other previous works from Iraq and the Middle East regions the following ages were proposed for the studied formations: Shiranish Formation: Late Campanian Maastrichtian. Hartha Formation: Late Campanian Early Maastrichtian. Mushorah Formation: Early Campanian.


INTRODUCTION
A total of 300 core and cutting samples were obtained from the Upper Cretaceous successions of four boreholes ( A, B, C and D ) located at theWest of Tigris river in the Northwest and central Iraq (Fig. 1). These sections comprised three formations which are from younger towards the older: Shiranish, Hartha and Mushorah Formations. In this respect (Figs. 2,3,4 and 5) show the stratigraphical sections of the above formations in the studied boreholes. Geographically the studied area is located Northwest and central Iraq approximately between latitude 34° 20´ -35° 50´ N and longitude 42° 50´ -43° 40´ E. Geologically, the investigated area belongs to the unstable shelf of Iraq and situated within Hamrin-Makhul subzone and Tigris subzone (Buday and Jassim,1987). Regionally it is situated between two main Phanerozoic units of the Middle East (the Arabian part of African platform on the west and the Asian branch of the Alpine geosyncline on the east) (Buday, 1980;Buday and Jassim, 1984).

AIM OF STUDY
The main aim of the present study is to identify the ostracode biozones of Shranish, Hartha and Mushorah Formations and their ages in the studied area.

BIOSTRATIGRAPHY
In Iraq, the biozonations by ostracode have received little attention from micropalaeontologists, due to the fact that most of the works have been done on the foraminifera by the oil companies. Therefore there is a shortage of literature dealing with the ostracode biozones in the stratigraphy of Iraq. However this research represent an attempt to use the ostracode species in the definition of biozones on the studied formations.The stratigraphic range of ostracode species in the studied formations from four oil boreholes are illustrated in (Figs. 6,7,8 and 9).   (Al-Sheikhly, 1982). Age: Early Campanian Boundaries: The base of this zone is defined by the first appearance of Cytherella IRC22 (Grosdidier, 1973) whilst the top of this zone is marked by the first appearance of Occultocythereis elongata (Al-Sheikhly, 1982) coeval with disappearance of the following species: Cytherella sp.1 Babinoti et al., (1988 ), Cytherella sp.1 Colin et al., ( 1982 ), Brachycythere IRC28 Grosdidier (1973 ). Associated species: The following species make their first appearance in this zone and extended into the overlying zones: Bythocypris gohrbdonti Esker ( 1968), Bythocypris windhami Butler and Jones (1957), Paracypris sp.A Esker (1968), Argilloecia taylorensis Alexander (1935) and Abyssocypris adunca Esker (1968).
Other species which occur in this zone and make their last appearance within it but were also recorded from the lower zones are: Cytherella sp. 1 Babinoti et. al., (1988). Cytherella sp.1 Colin et al., (1982 ) and Brachycythere IRC28 Grosdidier, ( 1973).
Only one species Cytherella cf. OUM 1110 Grekoff (1969), was recorded from this zone although it also occur in the overlying and underlying zones. In comparison with foraminifera, This zone is equivalent to Globotruncana fornicata, Globotruncana elevata and Globotruncana stuartiformis biozone of Al-Jassim et al., (1989 ) from North Iraq. It also equivalent to Globotruncana elevata biozone or Globotruncana stuartiformis of Hammoudi, (1995) from Central and South of Iraq.
In the present study the occurrence of Hartella inflatotriangularis and Hartella triangularis in this zone makes it is equivalent to the Problematicum Ms1 biozone of Falhe et al., (1987) from North Iraq. Problematicum Ms1 Hart is synonymous with Hartella inflatotriangularis, therefore this zone is also equivalent to Hartella inflatotriangularis zone of Al-Eisa and Al-Fassola, (2011) from NW Iraq. Biozone 2: Occultocythereis elongata Interval zone Definition: Interval zone represents the restricted range between the first appearance of Occultocythereis elongata Al-Sheikhly, (1982) and the first appearance of Krithe sp. M1113 Donze, ( 1973.

Age: Late Campanian
Boundaries: The base of this zone is characterized by the first appearance of Occultocythereis elongata Al-Sheikhly, (1982) whilst the top is marked by the first appearance of Krithe sp. M1113 Donze, ( 1973. Associated species: Ostracode species Cytherella sp.2 Colin et al., (1982) and Bairdia IRC19 Grosdidier, (1973) makes their first appearance in this zone and they are restricted to it.

Cytherella (C.) sylvesterbradleyi
Regionally, this zone is correlated to the Peleriops phumatoides biozone of Athersuch, (1994) from Arabian Gulf. Biozone 4: Holcopocythere bassiporosa Interval zone Definition: Interval zone represents the restricted rang between the first appearance of Holcopocythere bassiporosa Al-Furaih, (1980) and the last occurrence of Paracypris jonesi Bonnema, (1940). Age: Late Maastrichtian. Boundaries: The base of this zone is defined by the first appearance of Holcopocythere bassiporosa Al-Furaih, (1980), while the top is determined by the last occurrence of Paracypris jonesi Bonnema, (1940). Associated species: Two species appear first within this zone and were restricted to it. These are: Isohabrocythere teiskotensis and Xestoleberis sp.
Distribution and thickness: This zone is recorded from the studied boreholes except borehole B. It is confined to the upper parts of Shiranish formation where the thickness reaches to: 109 m, 119 m, 207m in A, C and D boreholes respectively. Discussion: This zone represents the youngest in the studied sections. The top of this zone is observed by the conspicuous rarity of most ostracode species and by the disappearance of Paracypris jonesi Bonnema, (1940). while the bottom is characterized by the obvious occurrence of the species Gansserina gansseri (planktonic foraminifera) beside some of ostracod species. In addition, the top of this zone is taken as are unconformity which separates Shiranish Formation (Mesozoic) from Aaliji or Jaddala Formations (Cenozoic). Correlation: This biozone is equivalent to Hornibrookella divergensis biozone of Athersuch, (1994) from Arabian Gulf. According to the same author, the index species Holcopocythere bassiporosa Al-Furaih, (1980). which is recorded in the present study resembles both Holcopocythere bassiporosa Al-Furaih, (1980) and Holcopocythere falsosulcata Al-Furaih, (1980) Northern Iraq. Raf. Jour. Sci., Vol. 11, No. 4, pp. 50 -58. Henigstein, A., Raab, M. and Rosenteld, A., 1985